Adapting maritime structures to rising sea levels

Sea level rise has accelerated over the last century under the effects of climate change. A few millimetres of annual rise may seem insignificant to a non-expert, but it means global sea levels will increase inexorably by several tens of centimetres between now and the end of the twenty-first century. This will worsen erosion, river flooding and coastal flooding, heightening the threat that already hangs over ever-growing populations and infrastructure in coastal and maritime areas. Factoring this sea level rise as accurately as possible into the design of developments – protection structures in particular – means implementing new approaches and methodologies.

Franck Mazas, a metocean expert in Artelia’s maritime and coastal team, explains.

What is sea level rise, and why is it accelerating?

Climate change is heating the ocean surface layer – and causing it to expand – while also melting inland freshwater resources such as glaciers and the polar ice caps. The combination of these two phenomena is driving sea level rise. During the twentieth century the annual rate of rise was just 1 or 2 mm, but nowadays it is reaching 4 to 5 mm. Due to the inertia in our climate system, this increase is set to continue; the only uncertainty is by how much. The extent will depend on the greenhouse gas emissions reductions that humanity is able to achieve.

Scientists now estimate that sea levels will probably rise by 50 to 60 centimetres by the end of this century, but the worst-case scenarios put this figure at more than a metre, with a marked acceleration after 2050.

What are the main risks associated with this phenomenon?

Sea level rise is heightening flood risks, changing river flows and estuary hydrosedimentary dynamics, and directly contributing to increased coastal flooding during storms. It is also altering how waves are transformed, causing them to break further inshore and making it easier for them to attack the coast, even in normal weather conditions. This increase in wave action will fuel the erosion of both sandy and rocky coasts. Rising sea levels can also increase the salinity of land and coastal freshwater resources. They put coastal developments of all kinds under greater pressure. According to the latest assessments, sea level rise could affect nearly a billion individuals and cause damage estimated at more than 10% of global GDP by the end of the century.

How can we adapt to these heightened risks?

Climate change adaptation in coastal zones is becoming a vital issue, and being addressed in national strategies. There are several possibilities. The first is the “conventional” approach, which involves providing even more protection by raising the heights of structures and ensuring that they can withstand harsher conditions. In parallel, nature-based solutions are increasingly being adopted. These consist in allowing more room for the sea and for river estuaries, by agreeing to remove polders, for example, and conserving or restoring mangroves. Other adaptation measures might include building on piles, or carrying out drainage or pumping operations. And a retreat may even have to be considered in some cases, as humans have built extensively in areas that are set to become uninhabitable. These are political decisions which entail rehousing inhabitants.

Artelia is frequently involved in projects to adapt coastal areas to climate change, by analysing their vulnerabilities in light of the changes taking place and implementing the most suitable solutions. In many cases, such as the sustainable development of the Somme bay (France) or the stabilisation of the Grand-Lahou tidal inlet (Ivory Coast), this involves combining several solutions within a given area. The situations can be highly complex, as is the case in Mayotte where, in the context of protecting part of the coastline and building the Petite Terre maritime terminal, we have to consider sea level rise, ground sinking, and a very narrow coastal fringe all at the same time.

How is sea level rise affecting the way you design maritime structures?

These structures will have to last several decades, or even a century in some cases, and will be very significantly affected by rising sea levels, especially towards the end of their lifetime. So we have to factor this phenomenon into their dimensional design, if they are to keep people and property safe.

Up to now, we have always considered sea levels, waves and winds to be constant when assessing hazards on probabilities of occurrence of 1 to 2% each year (a return period of 50 to 100 years), and sizing maritime structures accordingly. Artelia has long developed some probabilistic methods for determining the sea levels and associated wave heights likely to undermine the stability of a structure or generate overtopping.

We are still using these methods, but are adapting them because we now have to bear in mind that the structure will operate in increasingly harsh conditions as the years go by, increasing the probability of exceedance or more extensive damage, and reducing the return periods.

Can you tell us more about how Artelia is proposing to adapt its methods?

The easiest way to address sea level rise is to use the situation at the end of the structure’s lifetime as the baseline. For example, if we consider that the level will have risen 60 cm a hundred years from now (one of the intermediate scenarios proposed by the IPCC), we increase the structure height accordingly. But this approach means the structure will be oversized for most of its lifetime, leading to all the issues associated with consuming more materials, a higher economic cost, a bigger environmental footprint and impact on the landscape, etc.

To avoid this systematic oversizing, which is not exactly sustainable, we have developed a new approach that involves assessing the year-on-year changes in metocean conditions in greater detail, in order to optimise the structure in light of the associated risks. We start from a given climate change scenario and calculate the differences in probability in order to determine an intermediate value for sea level rise. This then enables us to size the structure while maintaining a risk level equivalent to the one that had been accepted up to then.

How was this method developed?

It is the fruit of a long tradition of research within the Artelia Group, nurtured by regular exchanges with the academic scientific world and through our clients’ projects. Tying in with this approach, in 2017 Artelia supported a research-based PhD which determined the wave/water level combinations to be considered when sizing structures, using a method that was applied practically for the first time in 2018 to the extension of La Cotinière harbour. Then, when we were commissioned to build an additional terminal in the port of Abu Dhabi, we used this method again in order to integrate sea level rise into the dimensional design of both the protection structures and the quays. Oversizing the quays to take into account a probable sea level 100 years from now raised different issues, especially from an operational standpoint. So we fine-tuned our calculation methods, and in the end we used this methodology to determine the quay elevations.

Do you use it routinely today?

We are implementing it in several projects. In Malta, for instance, to protect the harbour of Valletta – a UNESCO World Heritage Site – we were able to propose protective and energy-dissipating structures flush with the water surface. This method enabled us to strike the best compromise between hydraulic efficiency and a reduced visual impact. We are also using it at the moment to study the restoration of another emblematic site: Fort Boyard in France. This is a project spearheaded by the Charente-Maritime departmental council, aiming to rebuild the protective spur at the front of the fort and the berthing structures at the back [lien ?[1]]

This new methodology, which consists in assessing the hazards over the entire lifetime of a structure, offers us another advantage: it can be used to examine adaptive structures such as stepped quays or raisable crest walls, which can be easily reworked and modified as sea levels increase.

Written by par Eric Robert, published on 09 june 2024.